Tesla Coils - Frequently Asked Questions
What is a Tesla coil?
A Tesla coil is a device that uses resonant circuits and alternating current to produce extremely high voltages. Originally invented by Nikola Tesla in the late 1800s, Tesla coils have progressed from spark-gap circuits to designs involving modern solid-state switching devices such as MOSFETs and IGBTs. While there are many types of Tesla coils, what they all have in common are air-cored induction coils. Using a Tesla coil is the best way to produce a continuous high-voltage streamer.
Some Tesla coils can be modulated to play music using the lightning they produce. At first it can be hard to believe that sound is coming from the streamers themselves, but it’s true, there is no speaker attached to a singing Tesla coil!
Note: A Tesla coil is not the same thing as a Van de Graaf generator, though they are sometimes confused because they are both popular methods of making high voltage. A Van de Graaf generator uses a rotating belt to separate charges between ground and a metal terminal. A Tesla coil does not accumulate static charge and is AC, not DC electricity.
What can you do with a Tesla coil?
Tesla coils don’t really have any practical application other than looking and sounding awesome! Certain solid-state Tesla coils can be modulated to play music, and some musicians have used Tesla coils in performances.
Demonstrations that you can do with a Tesla coil include:
- Wirelessly excite fluorescent tubes
- Light an LED remotely
- Show how conductive plasma is by letting the coil’s arc jump through a flame
- Demonstrate the skin effect by showing how a metal cage shields a fluorescent tube inside of it
- Slowly turn up the repetition rate on a singing coil to explain the concepts of sound and frequency
- Play your favorite songs on a MIDI keyboard and hear them produced by lightning!
What are the various types of Tesla coils?
Tesla coils fall into two categories: spark-gap coils and solid-state coils. Every Tesla coil consists of a primary LC circuit that excites a secondary circuit. Solid-state and spark-gap coils vary in how they drive the primary sides of the coil. Solid-state coils have a number of common sub-varieties as well.
Spark Gap Tesla Coil (SGTC)
Spark gap coils use an air gap to control the primary current. Using a transformer (often a neon sign transformer or “NST”) , a primary capacitor is charged to a high voltage. When the voltage is high enough, the spark gap breaks down, ionizing the air between the terminals and forming a short circuit. This allows current to flow between the primary capacitor and primary inductor, completing the primary circuit. Power is lost to dissipation in the coils due to their resistance, and the spark gap is soon extinguished. Then the primary is slowly recharged and the cycle starts again.
Solid-State Tesla Coils
Solid-State Tesla Coils (SSTCs) encompass all Tesla coils which use a semiconducting device(s) to generate the RF power for the secondary. The consist of several types:
Single-resonant coils couple RF power (usually at a few hundred volts peak to peak) to the secondary via a single coil. These coils are best known for their bushy, quiet sparks and their high continuous RF power. However, their spark length to power efficiency metric is poor (8” of sparks typically requires a thousand watts or more).
Interrupted Solid-State Tesla Coils (ISSTC): The precursor to the DRSSTC, these interrupt the drive signal to an SSTC in order to reduce power consumption while preserving spark length. Generally the preferred way to build an SSTC unless you are after the bushy, silent look (or high-fidelity audio reproduction), as it is far easier to thermally manage than a continuous SSTC and less temperamental than a DRSSTC.
Class-E Tesla Coils: These use a Class-E inverter and a secondary operating at several MHz. The higher frequencies (which are enabled by the extremely efficient Class-E topology) allows for a stable, silent spark, which can consequently be used to reproduce full-range audio. However, typical variants use a low (~100V) bus voltage and consequently have very poor spark performance relative to other coil types (2-3” at best), while off-line ones require careful tuning and high powers to produce 4-5” of spark. Despite these shortcomings, this type of coil cannot be beat for its audio quality.
Dual-Resonant Solid-State Tesla Coil (DRSSTC)
The DRSSTC produces the longest sparks of the solid-state coils; in fact, DRSSTCs are approaching the performance of large spark-gap coils while offering a significantly more compact driver and full electronic control. The primary circuit of a DRSSTC is tuned to the same frequency as its secondary. In this way it can achieve a very high voltage on the primary and transfer a lot of energy to the secondary.
What makes a Tesla coil “sing”, or play music?
It may be hard to believe that you can make music with lightning, but the sound is indeed coming from the spark!
Sound is a pressure wave, which in a conventional loudspeaker is created by the vibration of a speaker cone. The range of human hearing is between about 20 Hz and 20,000Hz, so a speaker vibrating above 20,000Hz cannot be heard, because it is above the audible range. Sound can just as well be created by the pulsation of a plasma streamer. A single streamer sounds like a loud snap—just like a single jerk of the loudspeaker cone. If you repeat those snaps fast enough, they sound higher and higher in pitch. The snaps in a Tesla coil are repeated so fast that they are above the level of human hearing. To create an audible tone, the intensity of the sparks, which are firing away at high frequency, are modulated at the frequency of the tone. So to play middle C, the sparks’ intensities pulsate at about 262Hz. The interrupter is the device in charge of pulsating the spark in this way to play musical tones.
What does “polyphony” mean and how is it done?
Polyphony is playing multiple notes at once, as opposed to monophony which is only one note playing at once. Most music is polyphonic. One technique for generating polyphonic music on Tesla coils is to use multiple coils at the same time, each playing one note, to together create a multi-tone track. Another technique is to interleave the pulses that control the on-times of the Tesla coil and the corresponding audio frequencies, thereby creating two notes at once.
What is MIDI?
Typically, musical Tesla coils are controlled via a protocol called MIDI. MIDI files are save individual notes, and have indicators of which tones those notes are supposed to take. For example, if you had a piece with a piano and a violin, the piano notes would be saved in one track, and the violin would be saved in another. How the piece would actually sound depends on what you use to play back the track, because not all computer-generated pianos and violins sound the same.
Why are most DRSSTCs only capable of playing a maximum of two notes at once?
DRSSTC’s have fundamental limitations on the duty cycles and pulse widths that can be run. As more and more notes are played, the pulse widths get longer and longer. Hardware is needed to truncate the pulse widths when they get too long in order to avoid damaging the power electronics, which results in a severe loss of quality. For this reason, most DRSSTCs choose to run one or two notes simultaneously.
What can you learn by building a Tesla coil?
Building a Tesla coil will teach you universally-useful skills. A Tesla coil can give you a strong intuition of how high-frequency circuits work; designing a good Tesla coil resonator incorporates key RF concepts such as resonant tank circuits and quality factor. The use of high voltage is important in an array of fields such as particle accelerators and lasers. Tesla coil drivers operate on several core power electronic structures, such as the zero-current switched solid state inverter, and building one will teach you about power transistor behavior and non-idealities. Mounting a printed circuit board and winding the Tesla coil primary will require you to develop some mechanical dexterity and an ability to think of how things fit together. Finally, modern solid-state Tesla coils are fairly complex systems, and by successfully constructing one, you will learn how to efficiently debug circuits.
How dangerous is a Tesla coil?
A Tesla coil poses many hazards, and taking safety precautions are an absolute must. The hazards can be lessened by being a careful worker, keeping a workspace clear of clutter, wearing safety glasses when the board is energized, ensuring that the capacitors are discharged before attempting work on the board, keeping sensitive electronics and flammable objects away from the coil when it’s running, and generally being intimately aware of the operation and hazards of the Tesla coil.
DO NOT operate the Tesla coil near persons with medical implants such as pacemakers.
DO NOT operate the Tesla coil around any sensitive electronic devices.
DO NOT do any high voltage work alone
DO NOT do high voltage work when you are tired, intoxicated, or otherwise unable to give it your full attention
DO NOT build a Tesla coil if you don’t feel confident in your soldering and debugging skills
What are some other resources for Tesla coil design?
Steve Ward’s site: One of the leaders in DRSSTC design; some of his specific instructions are somewhat outdated, but nevertheless it remains a great resource.
Richie’s Site: A wealth of SSTC analyses and information; one of the few hobbyist-level walkthroughs of inverter operation.